When used with marine propulsion systems, a drive shaft or propeller shaft (prop shaft) is a mechanical component used for transmitting torque and rotation to one or more marine propellers. The drive shaft connects components of the drive train, such as the marine motor, that cannot be connected directly to the propeller because of distance needed for allowing relative movement between these components. As a torque carrier, the drive shaft is subject to torsion and shear stress, equivalent to the difference between the input torque and the load. The drive shaft must be strong enough to bear the stress, whilst avoiding too much additional weight that would in turn increase its inertia.
On a power-driven boat or ship, the drive shaft usually connects the transmission inside the vessel directly to the propeller, passing through a stuffing box or other seal at the point it exits the hull. The shaft connects through the propeller where is it fastened with a keeper nut at its end. The shaft typically includes a keyway, so that a key may be wedged into the keyway for tightly engaging the shaft into the hub of the propeller. A problem often occurs during installation if the key does not seat properly within the keyed slot of the propeller and the keyed slot of the shaft. Should the key slide forward and ride up the radius of the keyed slot in the shaft, it will create a pinching of the key and the keyed slot of the propeller forcing non fit/misalignment thus causing drive shaft vibration, loss of torque and drive shaft strength.